The electrode plate of a lead-acid battery consists of the grids, made of lead or lead alloy, onto the meshes of which an active material is filled. Grids can be produced, apart from the method of forming them directly into the grid shape by e.g. casting lead or lead alloy, with the method of forming meshes with an expander on the lead sheet made of lead or lead alloy. There are two types of expanders: the reciprocal type, which forms meshes sequentially from each end of the lead sheet by vertical motions of a dice cutter, and the rotary type, which employs a disk cutter that in rotation forms zigzag pattern slits on the lead sheet. In the latter type this lead sheet is then stretched toward both of its ends, expanding slits onto the grids. A grid that has been produced with the rotary method is called the rotary expanded grid. The device of the rotary type that produces grids is called a rotary expander.
As FIG. 7 shows, in the middle disk cutter (1) of a disk cutter used for the rotary expander, a number of ridges that are relatively long in their circumferential direction (1a) and a number of valleys that are relatively short (1b) are alternately placed at the periphery of a metal disk along this circumferential direction. Each of the ridges (1a) has a peripheral side face formed that protrudes toward the outer periphery further from reference circumferential face with a predetermined radius from the center of axle of the middle disk cutter (1). This reference circumferential face is planimetrically shown in the enlarged picture in the oval shape of FIG. 7. A peripheral side face with this reference circumferential face is formed for each of the valleys (1b). Also, for each of the valleys (1b), a groove (1c) that opens on the peripheral side face of the corresponding valley (1b) is formed. Each groove (1c) is found on the side of the disk opposing to the adjacent grooves (1c) across a ridge (1a). This is to say that on each side of the middle disk cutter (1), there is a groove (1c) for every other valley (1b). This also means that the valley (1b) with a groove (1c) on one side of the disk and the valley (1b) with a groove (1c) on the opposite side of the disk are adjacent to each other, alternately placed along the circumference. Generally, these grooves (1c) are formed on a disk side of the middle disk cutter (1) and have approximately the same size in width as the circumference of the valleys (1b) in the circumferential direction, as well as approximately half the size of the thickness of the middle disk cutter (1) in depth in the thickness direction of the disk. These grooves (1c) open at the valleys (1b) on the peripheral side face. Furthermore, the widths of the grooves (1c) of a middle disk cutter (1) in the radial direction are of a fixed length toward the center of axle from the peripheral side face.
A plurality of the above-described middle disk cutters (1) constitute disk cutter rolls (2), by being fixed in juxtaposition on the shared rotation shaft, separated from each other by such means as spacers (not displayed) with approximately the same distance as the thickness of the middle disk cutter (1). Two of these cutter rolls (2) are then placed opposite to each other vertically (or horizontally), as shown in FIG. 8. A lead sheet (3) is then inserted therebetween along the metal sheet conveyor guide (5), forming a number of zigzag slits (3a) (NB: there are also cases where three or more, instead of two, disk cutter rolls (2) are used). In this process, the top and bottom middle disk cutters (1) are placed so that the valleys (1b) will only slightly overlap with each other, as well as shifted by half a pitch toward the axis so that each of the top middle disk cutters (1) can be between the bottom middle disk cutters (1), as shown in (a) and (c) of FIG. 9. Also as shown in FIG. 9(a), the phase in the rotating direction is adjusted so that when the valley (1b) of the bottom middle disk cutter (1), with a groove (1c) on one side (right side in the Figure) of the disk, reaches the top edge, the valley (1b) of the top middle disk cutter (1), with a groove (1c) on the opposite side (left side in the Figure), reaches the bottom edge. Therefore, as shown in FIG. 9(b), the ridge (1a) of the top middle disk cutter (1) reaches the bottom edge when the ridge (1a) of the bottom middle disk cutter (1) reaches the top edge. Furthermore, as shown in FIG. 9(c), when the valley (1b) of the bottom middle disk cutter (1), with a groove (1c) on one side (left side in the Figure) of the disk, reaches the top edge, the valley (1b) of the top middle disk cutter (1), with a groove (1c) on the opposite side (right side in the Figure), reaches the bottom edge.
This set of two disk cutter rolls (2) combined and opposed to each other is called a disk cutter cluster. At each end of a disk cutter cluster, an edge disk cutter (4) is attached (in FIG. 9 one edge disk cutter (4) is placed on each side of the bottom disk cutter roll (2)). On the periphery of an edge disk cutter (4), ridges (4a) and valleys (4b) are alternately placed, as in FIGS. 10 and 11. The compositions of the valley (4b) and of the groove (4c) formed on this valley (4b) is the same as those of a middle disk cutter (1) valley (1b) and of its groove (1c), but on the ridge 4a, a peripheral side face consisting of a reference circumferential face is formed. Namely in this edge disk cutter (4), the ridge (4a) does not protrude toward the outer periphery, nor does the valley (4b) take a relatively dented shape compared with this ridge (4a). Such an edge disk cutter (4) is placed at each end of a bottom disk cutter roll (2), in such a way that it will become adjacent to and outside the middle disk cutter (1) at each end of the top disk cutter roll (2).
When a lead sheet (3) is guided to the disk cutter cluster of the above-described composition along its metal sheet conveyor guide (5), the lead sheet (3) is cut as the ridges (1a) of the top and bottom middle disk cutters (1) overlap, as shown in FIG. 9(b) and FIG. 8. As a result, slits (3a) are formed on the lead sheet (3). Furthermore, thin and long wires (3b), formed between a plurality of slits (3a) adjacent to each other on the lead sheet (3) in the width direction, protrude vertically in a ridge-like shape from the sheet surface of the lead sheet (3) alternately, as they are pushed by the top and bottom ridges (1a). Thus, as shown in FIGS. 9(a) and (c) as well as in FIG. 8, slits (3a) are continually formed as the lead sheet (3) is being cut at the adjacent valleys (1b), at which the grooves (1c) of the top and bottom middle disk cutters (1) are faced back to back, as the peripheral side faces of the valleys (1b) slightly overlap with each other. On the other hand, the lead sheet (3) is not cut where the grooves (1c) are faced head on at the adjacent valleys (1b), as the peripheral side faces of the valleys (1b) do not overlap. Here nodes (3c) are formed instead of slits (3a). Thus slits (3a) of the length twice the size of a ridge of the ridge-shaped wires (3b) pushed by ridges (1a) are formed on the lead sheet (3), while no slit is formed on nodes (3c). Namely these slits (3a) and nodes (3c) are alternately formed continually toward the direction in which the sheet is conveyed. Also, those slits (3a) adjacent on the lead sheet (3) in the width direction show a zigzag pattern as in the circled horizontal projection in FIG. 8, as nodes (3c) are formed with half a pitch shifts.
As shown in FIG. 9(b), the ridges (4a) of edge disk cutters (4) in the bottom disk cutter roll (2) overlap with the ridges (1a) of middle disk cutters (1) at each end in the top disk cutter roll (2).This causes the lead sheet (3) to be cut, slits (3a) to be formed and wires (3b) to be protruded downward in a ridge shape. The valleys 4b and 1b slightly overlap with each other also at the adjacent part where the grooves (4c) at the valleys (4b) of the bottom edge disk cutters (4) at both ends are faced back to back with the grooves (1c) at the valleys (1b) of the top middle disk cutters (1) at both ends, as demonstrated in FIG. 9(a) (right end) and in FIG. 9(c) (left end). This causes the lead sheet (3) to be cut and slits (3a) to be continually formed. However, the peripheral side faces of the valleys 4b and 1b do not overlap with each other at the adjacent part where the grooves (4c) at the valleys (4b) of the bottom edge disk cutters (4) at both ends are faced head on with the grooves (1c) (the left end in FIG. 9(a) and the right end in FIG. 9(c)), due to the grooves 4c and 1c. Thus the lead sheet (3) is not cut and instead the edge node (3d) is formed that is similar to a node (3c). It should be noted that slits (3a) are not formed at the outer end of the edge node (3d): this means that the edge node (3d) is directly joined with the frame portion (3e), which is formed at each end in the width direction of the lead sheet (3).
The lead sheet (3) with a number of slits (3a) formed on it in the above-described manner is then stretched toward both ends in the width direction in a post-process. Consequently these slits (3a) are expanded so as to form meshes, as shown in FIG. 12. Namely, a grid is formed whose nodes (3c) and edge nodes (3d) are connected by means of wires (3b). Attention is herewith drawn to the fact that, while in reality each node (3c) is pulled by the wire (3b) in expansion and leans toward the direction in which the wire is contorted, simplified FIG. 12 does not show such contortion.
Also noted is that while edge disk cutters (4) are installed at both ends of the bottom disk cutter roll (2) in the case discussed above, it is possible that edge disk cutters (4) are installed at both ends of the top disk cutter roll (2), or that one edge disk cutter is installed for the bottom disk cutter roll (2) while the other is for the top disk cutter roll (2).
In some cases, a plurality of disk cutter rolls (2) are installed for the same rotation shaft, resulting in a plurality of disk cutter clusters for a pair of rotation shafts. This makes it possible for grids to be formed simultaneously in a number of rows. In this case, edge disk cutters (4) will be installed for both ends of each disk cutter cluster.
In the above described nodes (3c) and edge nodes (3d) of a lead sheet (3), as shown in FIGS. 9(a) and (c), their both ends in the width direction (horizontal direction in the Figure) are pressed by valleys (1b) of a middle disk cutter (1) or valleys (4b) of an edge disk cutter (4), where their grooves (1c and 4c) are faced head on, in the vertically opposite directions. Therefore, deformation in the vertical direction that is no smaller than the lead sheet (3) thickness is caused at both ends in the width direction in nodes (3c) or in edge nodes (3d). Furthermore, the intervening parts of the two slits (3a) juxtaposed in the length direction becomes thinner as a result of this deformation in nodes (3c) or in edge nodes (3d).
A problem has been observed for a lead-acid battery that uses the conventional rotary expanded grid produced with the conventional rotary expander described above: heavy corrosion takes place in the edge nodes of a grid when charge-discharge is repeated. This leads to another problem, namely a reduction in the electric current collection capability of a grid. This is because the cross-section area of the electrically conductive part of an edge node (3d), which has already been thinned in expansion, becomes susceptible to be reduced further in size due to the corrosion. Furthermore, more heat is generated when the cross-section area of the electrically conductive part of an edge node (3d) is reduced in size, due to an increased current density in charge-discharge. This leads to the problem of an increased likelihood of meltdown in the said part. In particular, the electric current collection performance of a grid deteriorates considerably when a rupture is caused at the edge node (3d) that is connected with the frame, among the frames at both ends (3e), where a lug is formed, as it collects electric currents of an electrode plate. There also is a problem in the case that such a rupture is caused at the edge node (3d) that is connected with frames (3e) where a lug is not formed. If rupture occurs in a plurality of such edge nodes (3d), these frames (3e) could dangle from electrodes to a position where separators are not opposed. Thus short circuitry could ensue between the electrodes with different polarities. The corrosion of a grid of the above described kind has been an extremely serious problem for negative electrodes rather than for positive electrodes.
The reason for the concentrated corrosion in the edge node of (3d) a grid described above is not entirely clear, but can be inferred as follows. In a grid, the parts where great contortion is caused in the slit (3a) forming process with disk cutter clusters and the expansion process thereafter are edge nodes (3d) and nodes (3c). Among these parts, nodes (3c) are stretched by wires (3b) in expansion and incline toward the direction in which the wires (3b) are contorted. Therefore, the contortion formed at the nodes (3c) when expansion occurs is restricted just by the amount of this inclination. In contrast, the edge node (3d) cannot incline toward the wire (3b) contortion direction when expansion occurs, as it is fixed to the frame (3e). As a result, greater contortion and more minute cracks are caused in expansion at edge nodes (3d) than non-edge nodes (3c). It is presumed that as a result of this, this part of the apparatus tends to become corroded more easily at the time of charge-discharge of a battery.
Apart from the corrosion problem in a grid, another problem has been observed concerning rupture that occurs in a grid by such factors as fatigue due to vibration, when the battery is exposed to vibration in such uses as in a mobile device. Such rupture also tends to occur at edge nodes (3d), where higher likelihood of greater contortion and minute cracks is observed in the case of corrosion. In a lead-acid battery, the mechanical strength is low for the grid of the negative electrode, which is thinner than that of the positive electrode, as the negative electrode is normally formed more thinly than the positive electrode. Consequently the problem of fatigue caused by vibration has been more serious for the negative electrode than the positive electrode.